As hydraulic transmission systems, there have been known hydrostatic transmissions (HST) that convert all the input power from the engine into hydraulic pressure for power transmission and hydro-mechanical (power split type) transmissions (HMT) that convert part of the input power into hydraulic pressure while mechanically transmitting the rest of it. Of these types of transmissions, the latter transmissions (HMT) are advantageous over the former transmissions (HST) since HMTs only convert part of the mechanical power into hydraulic power and therefore exert high mechanical power transmission efficiency. For this reason, HMTs are regarded as ideal transmissions for work vehicles subjected to significant load fluctuations such as bulldozers and wheel loaders and applied to some of such vehicles.
In a typical hydro-mechanical transmission (HMT), its infinitely variable speed characteristics are attained by a planetary gear train. Specifically, the planetary gear train has three elements, i.e., a sun gear, a carrier having planetary gears and a ring gear. Of these three elements, the first and second elements are coupled to the input shaft and output shaft respectively and the third element is coupled to a hydraulic pump or hydraulic motor. The rotational speed of the hydraulic pump or hydraulic motor is varied to change the rotational speed of the output shaft.
HMTs are classified into two types. One is the output split type in which a first pump/motor is coupled to a planetary gear train and a second pump/motor connected to the first pump through a hydraulic circuit is coupled to the input shaft of the transmission system at a constant rotation ratio. The other is the input split type in which a first hydraulic pump or hydraulic motor is coupled to a planetary gear train and a second hydraulic pump or hydraulic motor connected to the first one through a hydraulic circuit is coupled to the output shaft of the transmission system at a constant rotation ratio.
As a technique similar to HMTs, electro-mechanical transmissions (EMT) are known. EMTs use generators/motors in place of the pumps/motors used in HMTs and convert part of mechanical power into electric power for power transmission. A prior art technique associated with EMTs is disclosed in Patent Document 1. The transmission system disclosed in this document is an electro-mechanical transmission that has two planetary gear trains and two electric motors and is configured to be switched by clutches to establish an input split mode to provide a low speed range and a compound split mode to provide a high speed range.    Patent Document: U.S. Pat. No. 6,478,705
FIG. 16 shows a schematic diagram of a transmission system constructed according to Patent Document 1. The transmission system 100 shown in FIG. 16 has an input shaft 103 that inputs power sent from an engine 101 through a clutch 102; two planetary gearsets 104, 105 aligned coaxially with the input shaft 103; two generators/motors 106, 107 aligned coaxially with the planetary gearsets 104, 105; an output shaft 108 coupled to vehicle drive wheels (not shown) through a differential gearset (not shown); and a pair of selectively engageable clutches 109, 110. Each of the planetary gearsets 104, 105 is composed of a sun gear 111 (112); a plurality of planetary gears 113 (114) in meshing engagement with the outer periphery of the sun gear 111 (112); a carrier 115 (116) for supporting the shaft of the planetary gears 113 (114); and a ring gear 117 (118) in meshing engagement with the outer periphery of the planetary gears 113 (114).
Herein, the ring gear 117 of the planetary gearset 104 is connected to the input shaft 103. The carrier 115 of the planetary gearset 104 and the carrier 116 of the planetary gearset 105 are coupled to each other by the output shaft 108 so as to be rotatable together with the output shaft 108. The sun gears 111, 112 of the planetary gearsets 104, 105 are coupled to the rotors 106a, 107a of the generators/motors 106, 107 through sleeve shafts 119, 120, respectively, which are fitted on the output shaft 108. The clutch 109 is for connecting and disconnecting the ring gear 118 to and from the fixed end, whereas the clutch 110 is for connecting and disconnecting the ring gear 118 to and from the sleeve shaft 119. The stator 106b of the generator/motor 106 and the stator 107b of the generator/motor 107 are electrically connected to a storage battery 122 through an ECU (electronic control unit) 121.
In the transmission system 100, shifting between the input split mode and the compound split mode is effected at a vehicle speed at which the rotational speed of the generator/motor 106 becomes zero (this vehicle speed is hereinafter referred to as “mode switching point”). That is, if the current vehicle speed is within a vehicle speed range below the mode switching point, the clutch 109 is engaged and the clutch 110 is disengaged, thereby establishing the input split mode. On the other hand, if the current vehicle speed is within a vehicle speed range above the mode switching point, the clutch 109 is disengaged whereas the clutch 110 is engaged thereby establishing the compound split mode.
Setting of the mode switching point may be carried out in three patterns. In the first pattern, the mode switching point is set to a vehicle speed Vb that is just a half of a vehicle speed Vd at which the rotational speed of the generator/motor 107 becomes zero, as shown in FIG. 17(a). The second pattern is such that the mode switching point is set to a vehicle speed Vc that is higher than the vehicle speed Vb as shown in FIG. 17(b). In the third pattern, the mode switching point is set to a vehicle speed Va that is lower than the vehicle speed Vb as shown in FIG. 17(c). It should be noted that FIGS. 17(a) to 17(c) each show changes in the rotational speeds of the generators/motors 106, 107 in cases where the vehicle is accelerated in the forward direction with the rotational speed of the engine 101 being kept constant. In FIGS. 17(a) to 17(c), vehicle speed is plotted on the abscissa and the rotational speeds of the generators/motors 106, 107 on the ordinate. Solid line indicated by A represents the change of the rotational speed of the generator/motor 107 relative to vehicle speed and broken line indicated by B represents the change of the rotational speed of the generator/motor 106 relative to vehicle speed.
At the speed at which the rotational speed of the generator/motor 106 becomes zero (i.e., the mode switching point (Vb in FIG. 17(a); Vc in FIG. 17(b); and Va in FIG. 17(c)), the rotational speed of the generator/motor 106 is zero and therefore the engine power is not converted into electric power, so that all of the engine power is transmitted to the output shaft 108 through the mechanical mechanism alone. This “mode switching point” is also called “low speed side direct point”. Also, at the speed at which the rotational speed of the generator/motor 107 becomes zero in the speed range corresponding to the compound split mode (i.e., Vd in FIGS. 17(a) to 17(c)), the engine power is not converted into electric power but entirely transmitted to the output shaft 108 mechanically. This speed is hereinafter referred to as “high speed side direct point”.
In the planetary gearset 104, the ring gear (the third element) 117 is connected to the input shaft 103, the carrier (the second element) 115 is to the output shaft 108, and the sun gear (the first element) 111 is to the rotor 106a of the generator/motor 106. Therefore, if vehicle speed (the rotational speed of the output shaft 108) linearly changes with the rotational speed of the engine 101 being kept constant, the rotational speed of the generator/motor 106 will linearly change in all the modes, i.e., the input split mode and the compound split mode as indicated by broken line B in FIGS. 17(a) to 17(c). In other words, the rotational speed of the generator/motor 106 is directly affected by the rotational speed of the output shaft 108 throughout all the modes, i.e., the input split mode and the compound split mode.
Therefore, in the first pattern shown in FIG. 17(a), the rotating direction of the generator/motor 106 in the vehicle speed range of zero vehicle speed to the vehicle speed Vb (when the input split mode is selected) differs from the rotating direction of the generator/motor 106 in the vehicle speed range of the vehicle speed Vb to the vehicle speed Vd (when the compound split mode is selected). In addition, the relationship between the rotational speed Na of the generator/motor 106 at zero vehicle speed and the rotational speed Nb of the generator/motor 106 at the vehicle speed Vd is represented by Na=Nb. In the second pattern shown in FIG. 17(b), the rotating direction of the generator/motor 106 in the vehicle speed range of zero vehicle speed to the vehicle speed Vc (when the input split mode is selected) differs from the rotating direction of the generator/motor 106 in the vehicle speed range of the vehicle speed Vc to the vehicle speed Vd (when the compound split mode is selected), and the relationship between the rotational speed Nc of the generator/motor 106 at zero vehicle speed and the rotational speed Nd of the generator/motor 106 at the vehicle speed Vd is represented by Nc>Nd. In the third pattern shown in FIG. 17(c), the rotating direction of the generator/motor 106 in the vehicle speed range of zero vehicle speed to the vehicle speed Va (when the input split mode is selected) differs from the rotating direction of the generator/motor 106 in the vehicle speed range of the vehicle speed Va to the vehicle speed Vd (when the compound split mode is selected), and the relationship between the rotational speed Ne of the generator/motor 106 at zero vehicle speed and the rotational speed Nf of the generator/motor 106 at the vehicle speed Vd is represented by Ne<Nf.